Ahhh, the PacNW...oh how I miss it this time of year when my central air is on the fritz and it's 100 degrees in Michigan.
I'm actually flying out to Oregon (Portland and then the coast - can't wait) next week, flying back to MI, and then flying out to Seattle for a few days. This has to be the best time to be in the area, but to your question...
A M's game would be great, but if they're not in, I'd at least go check out the stadium.
How much 'free' time will you have? If you have a chance, it's about a 60-90 min drive up to Bellingham, which is a great spot. Around town there are many good restaurants, but even walking the downtown area, Pike's Place, the pier, Univ of Wash campus/grounds, Lake Washington, etc. Jump on a ferry and head over to Bremerton. Drive down to Olympia and then head West to the Peninsula (great places to get out of the car and breathe in the trees).
My time's been fun out here, but I could definitely move back.